In fields of mobile communications and speech storage, for effective utilization of radio signals and storage media, a speech coding apparatus is used that compresses speech information to encode at low bit rates. As a conventional technique in such a speech coding apparatus, there is a CS-ACELP coding scheme with DTX (Discontinuous Transmission) control of ITU-T Recommendation G.729, Annex B (“A silence compression scheme for G.729 optimized for terminals conforming to Recommendation V.70”).
FIG. 1 is a block diagram illustrating a configuration of a speech coding apparatus using the conventional CS-ACELP coding scheme with DTX control. In FIG. 1 an input speech signal is input to speech/non-speech determiner 11, CS-ACELP speech coder 12 and non-speech interval coder 13. First, speech/non-speech determiner 11 determines whether the input speech signal is of a speech interval or of a non-speech interval (interval with only a background noise).
When speech/non-speech determiner 11 determines that the signal is of a speech interval, CS-ACELP speech coder 12 performs speech coding on the signal of the speech interval. Coded data of the speech interval is output to DTX control/multiplexer 14.
Meanwhile, when speech/non-speech determiner 11 determines that the signal is of a non-speech interval, non-speech interval coder 13 performs coding on the noise signal of the non-speech interval. Using the input speech signal, non-speech interval coder 13 calculates LPC coefficients the same as in coding of speech interval and LPC prediction residual energy of the input speech signal to output to DTX control/multiplexer 14 as coded data of the non-speech interval. In addition, the coded data of the non-speech interval is transmitted intermittently at an interval at which a predetermined change in characteristics (LPC coefficients or energy) of the input signal is detected.
DTX control/multiplexer 14 controls and multiplexes data to be transmitted as transmit data, and outputs the resultant as transmit data, using outputs from speech/non-speech determiner 11, CS-ACELP speech coder 13 and non-speech interval coder 13.
The conventional speech coder as described above has the effect of decreasing an average bit rate of transmit signals by performing coding only at a speech interval of an input speech signal using a CS-ACELP speech coder, while at a non-speech interval (interval with only noise) of the input speech signal, performing coding intermittently using a dedicated non-speech interval coder with a number of bits fewer than in the speech coder.
However, in the above-mentioned conventional speech coding method, due to facts as described below, a receiving-side apparatus that receives data coded in a transmitting-side apparatus has a problem that the quality of a decoded signal corresponding to a noise signal at a non-speech interval deteriorates. That is, a first fact is that the non-speech interval coder (noise signal analyzing/coding section) in the transmitting-side apparatus performs coding with the same signal model as in the speech coder (generates a decoded signal by applying an AR type of synthesis filter (LPC synthesis filter) to a noise signal per short-term (approximately 10 to 50 ms) basis).
A second factor is that the receiving-side apparatus synthesizes (generates) a noise using the coded data obtained by intermittently analyzing an input noise signal in the transmitting-side apparatus.